Eratosthenes' observation on 21 June 2012

The workshops

On 21 june 2012 : Solstice celebration with Eratosthenes

Historical presentation

This event aims at a simple measurement which allows to determine the circumference of the Earth: Eratosthenes's measurement.
Eratosthenes is a Greek scientist who lived in the third century BC in Egypt. He observed that on the summer solstice at solar noon :

the sun was at its zenith in the city of Syene (Southern Egypt, now Aswan), so there was no shadow.

but in the North, in the city of Alexandria, the same day at the same moment, there was the shadow of an obelisk, which means the sun was inclined

Eratosthenes then measured in Alexandria the angle between the sun and the vertical. Then, knowing the distance between the two cities and assuming that, on Earth, the sun rays are parallel, he could set the earth's circumference.

The conferences

Conferences will be about the history of science and mathematics, and new knowledge about the sun. They will be broadcast live on the web.

We don't know how Eratosthenes did, but we can imagine how the ancient Greeks calculated by observing signs, thanks to their literature and archeology. Their verbal numbering had a 10 base, but because of their two ways of writing numbers, all written calculations were impossible! Thus they managed to calculate by arranging stones on a flat surface equipped with parallel lines: an abacus. That's how we can imagine how Eratosthene did his calculation.

11h – 12h15 "The sun: a magnetic star at the beginning of the Space Weather" Jean Lilensten, Observatory of Grenoble.

Our knowledge of the sun considerably changed over the last decade. Gradually, we have discovered the origin of its cycles of activity: the magnetic field is, of course, at the center of the various aspects of solar activity (electromagnetic flux, rashes, coronal mass ejection...). At the same time, we noticed that the Earth is surrounded by the field of wind and solar radiation. This has consequences on our technological societies, giving rise to a young discipline : space weather.

The measurement

2200 years later, we are going to reproduce the observations of Eratosthenes with our partners. The event in Lyon is mainly dedicated to pupils of primary and secondary schools. The students have to come with their teacher and the device they prepared for the measurement.
In the ENS de Lyon, the measurement will take place at 13h43 HLF (Ephemerides)

The 21 of June's videoconference

The 22 of June's videoconference

Classes who want to participate in this videoconference have to
1) Register on the Lamap website "Following in the foosteps of Eratosthene"
2) Send an email to charles-henri.eyraud(at)ens-lyon.fr, replacing (at) by @, with some details:
School : Name, Direction, Country, Latitude, Longitude. Age and number of students involved

Other measurements will have been done the previous days (19 and 20 June), and will be used by classes of America or, in case of bad weather, by French classes.

Other observations in the garden of the ENS de Lyon

Observation of sunspots

Observation of sun's corona

Participants will also be given the opportunity to make drawings of the sun and its spots as did Galileo. A drawing competition will be organised for scientific and artistic appreciation.

The images of the sun provided by webcams connected to telescopes will also be transmitted in real time.